Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters, such as ladder filters having electrically connected series and shunt resonators formed in a ladder structure. The filters may be included in a duplexer, for example, connected between a single antenna and a receiver and a transmitter for respectively filtering received and transmitted signals.
Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators. A BAW resonator, for example, is an acoustic stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers). Types of BAW resonators include a film bulk acoustic resonator (FBAR), which uses an air cavity for acoustic isolation, and a solidly mounted resonator (SMR), which uses an acoustic mirror for acoustic isolation, such as a distributed Bragg reflector (DBR). FBARs, like other BAW devices, may be configured to resonate at frequencies in GHz ranges, and are relatively compact, having thicknesses on the order of microns and length and width dimensions of hundreds of microns. This makes FBARs well-suited to many applications in high-frequency communications.
Resonators may be used as band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. The passbands of the resonator filters tend to shift in response to environmental and operational factors, such as changes in temperature and/or incident power. For example, the passband of a resonator filter moves lower in frequency in response to rising temperature and higher incident power.
Cellular phones, for example, are negatively affected by shifts in passband due to fluctuations in temperature and power. For example, a cellular phone includes power amplifiers (PAs) that must be able to deal with larger than expected insertion losses at the edges of the filter (duplexer). As the filter passband shifts down in frequency, e.g., due to rising temperature, the point of maximum absorption of power in the filter, which is designed to be above the passband, moves down into the frequency range of the FCC or government designated passband. At this point, the filter begins to absorb more power from the PA and heats up, causing the temperature to increase further. Thus, the filter passband shifts down in frequency more, bringing the maximum filter absorbing point even closer. This sets up a potential runaway situation, which is avoided only by the fact that the reflected power becomes large and the filter eventually settles at some high temperature.
In order to prevent or reduce frequency shift with rising temperatures, a conventional resonator used in a band-pass filter, for example, may include a layer of oxide material within the piezoelectric layer of the acoustic stack and/or one or more electrodes, an example of which is provided by Ruby et al., U.S. Patent Application Pub. No. 2011/0266925 (published Nov. 3, 2011), which is hereby incorporated by reference in its entirety. The oxide material has a positive temperature coefficient of elastic modulus over a certain temperature range. The positive temperature coefficient of the oxide material at least partially offsets the negative temperature coefficients of the metal electrodes and the piezoelectric material, respectively. For example, the oxide material may be placed in the center of the piezoelectric layer or at either end of the piezoelectric layer between the electrodes. However, the acoustic coupling coefficient (kt2) of the resonator is compromised by the addition of oxide material to the piezoelectric layer. This is because the oxide material appears as a “dead” capacitor in series with the active piezoelectric material dielectric. For example, when an oxide layer or other temperature compensation layer is inserted in an FBAR stack, the coupling coefficient kt2 becomes lower as the temperature coefficient of the FBAR stack becomes more positive.
Furthermore, the piezoelectric layer may be grown over the oxide material used for temperature compensation. The temperature compensation layer is generally an amorphous film, and therefore is not an oriented crystalline material. As such, the piezoelectric layer grown on certain known temperature compensation layers (e.g., silicon dioxide) will have a poor crystalline structure and a random mixture of c-axis orientations in the thin film, which prevents good piezoelectric response. By contrast, it is desirable to form a highly textured C-axis piezoelectric material demonstrating excellent piezoelectric properties. In order to provide a highly textured C-axis piezoelectric material, a seed interlayer may be applied to the surface of the electrode or the temperature compensation layer over which the piezoelectric layer is grown.
In addition, a seed interlayer on the temperature compensation layer may help to improve the positive temperature coefficient and the coupling coefficient kt2 of a temperature compensated FBAR structure. An example of a seed interlayer on a temperature compensation layer formed within a bottom electrode is provided by Zou et al., U.S. Patent Application Pub. No. 2014/0292150 (published Oct. 2, 2014), which is hereby incorporated by reference in its entirety. Generally, the thicker the seed interlayer, the more effective it is in providing highly textured C-axis piezoelectric material and improving the positive temperature coefficient, particularly with respect to a piezoelectric layer formed of commonly used aluminum nitride (AlN).
What is needed, therefore, is a temperature compensated acoustic resonator device that overcomes at least some of the noted shortcomings of known acoustic resonator devices described above.